Recently, a phenomenon has been discovered in which, once a voltage is applied to a specific metal oxide-based material, the material exhibits either of two conditions, which include a low resistance condition and a high resistance condition, depending on: a resistivity which the metal oxide-based material has before the voltage is applied: and the magnitude of the voltage applied. A new memory device using this phenomenon attracts attention. This memory device is termed as a ReRAM (Resistance Random Access Memory). From the standpoint of high integration, a three-dimensional cross-point structure in which memory cells are arranged at intersections between WLs (Word Lines) and BLs (Bit Lines) has been proposed as an actual device structure of the ReRAM (for instance, refer to JP-A 2009-021602 (Kokai)).
In the case of the three-dimensional cross-point structure, once a voltage is applied to a certain memory cell for the purpose of writing data to the certain memory cell, a reverse voltage is applied to unselected memory cells as well. For this reason, each memory cell needs to include not only a variable resistance film but also a diode. Examples of the diode in use to this end include a pin-type silicon diode obtained by laminating: a p-type silicon layer doped with acceptors; an i-type silicon layer doped with no impurity; and an n-type silicon layer doped with donors.
In the case of the ReRAM with the three-dimensional cross-point structure, for the purpose of integrating a larger number of memory cells on a single semiconductor memory device, diodes need to be miniaturized as well. However, this miniaturization decreases the area of each diode, and accordingly reduces an amount of forward current in the diode. For the purpose of preventing the malfunction of each memory cell, a certain amount of forward current flowing through the memory cell needs to be ensured. On the other hand, for the purpose of ensuring the reverse breakdown voltage, each diode needs to be securely equal to or longer than a certain length in the direction in which the current flows. This limits the reduction in the resistance by decreasing the length of the diode. For a similar reason, the reduction in the resistance by raising the concentration of an impurity is limited. These factors make it difficult to integrate an even larger number of memory cells on a single semiconductor memory device.